part 4] JURASSIC ciiROisroLoaT. 409 



the completely arenaceous conditions of the middle part of the 

 Bridport Sands. In this third episode Dumortieria ranges up some 

 60 feet in the Down-Clif£ Clay,i and it may range as far again 

 into the Bridport Sands before Catulloceras appears, while there 

 is about 170 feet before evidence of moorei is obtained. ^ 



The other supposition is that the upper part of the Watton 

 Bed was deposited contemporaneously with the Down -Cliff Clay. 

 This is possible, but it follows that a curiously limited horizontal 

 extension of the lithographic-stone deposit in a westerly direction 

 would have to be allowed, and, moreover, would have to be accounted 

 for ; because north-eastwards, so far as the Bothenhampton evi- 

 dence goes, there is reason to suppose considerable horizontal range 

 of lithographic -stone conditions. 



On this theory the non-sequence between the Watton Bed and 

 the sandy stratum might correspond to the time of deposition of 

 the upper part of the Down -Cliff Cla3^ 



So far, then, as our present time-scale allows, the three upper 

 layers of the Watton Bed were deposited in the earliest part of 

 the Dumortieria hemera. The layer below, which contains 

 Hammatoceras, could be dated as Hammatoceras hemera, which 

 precedes Dumortieria. The layer below (No. 5), which contains 

 striatulum forms, can be dated as certainly later than striatidum 

 hemera ; but whether it can be dated as contemporaneous is a 

 question for consideration later. 



In this layer I obtained a reversed gastropod {Cirrus^, thinking 

 it to be a new record ; but Moore has already figured and described^ 

 a similar form as Turho hertholeti from the ' Upper Lias at 

 Compton [near Sherborne, Dorset] . . . from the highest bed of that 

 place in association with Ammonites toalcottii.'' There is reason 

 to suppose at Compton and the neighbourhood a close association 

 of Grammoceras-striatulum forms with A. ivalcottii [cf. hifrons], 

 owing to paucity of sediment, erosion, or both; therefore the evi- 

 dence is not necessarily against this gastropod being a contemporary 

 of striatulum forms."* 



The peculiar Hildoceratid ammonites of layer 6 have some 

 resemblance to species which occur in the Jet-Rock of the York- 

 shire Upper Lias {exaratum hemera), as, for instance. Ammonites 

 rugatulus and A. multifoliatus Simpson ; there is also likeness 

 to A. similis Simpson, but this is perhaps from the falciferiim 

 horizon. The species of JElegantidiceras, JE. elegantulum and 

 ^. ovatulum (Simpson), 'Yorkshire Type- Ammonites ' ii, pis. xciii 

 & cvi, may also be cited as similar, but they are thicker : they are 

 from Jet-Bock, exaratum hemera. 



^ I, 5, p. 64. 2 i^ 5^ p_ 64_ 



^ VII, p. 210 & pi. vi, figs. 7-8. 



■* There is little doubt that, at Compton, Moore worked a higher (later) 

 deposit of Upper Lias than at Ilminster, Analysis of his finds should show 

 this. The forms common to Ilminster and Compton would presumably be 

 from the lower bed ; those peculiar to Compton might be expected to indicate 

 the higher bed. 



